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Old 20th January 2012, 03:36 AM   #11
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Location: Texas
Pic on post #6, I replace the traces that were cut (in pic they're RED) and now I have control of the input gain signal and can adjust so both right and left outputs are equal/same.

So problem is fixed and its working perfectly, will see if I can install it pretty soon.


Am still thinking if I should change/upgrade the two opamps (OP275GP), any suggestions? Am thinking OPA2134PA but dont know if there will be problems. Also so I wont be soldering and desoldering I will put IC sockets, I figured it would be a nice and easy mod.

Last edited by jeanious2011; 20th January 2012 at 03:40 AM.
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Old 20th January 2012, 05:52 AM   #12
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I think you might be splitting hairs by replacing those op-amps

Op275 spec:
Low Noise: 6 nV/÷Hz
Low Distortion: 0.0006%
High Slew Rate: 22 V/ms
Wide Bandwidth: 9 MHz

OP2134PA spec:
ULTRA LOW DISTORTION: 0.00008%
l LOW NOISE: 8nV/÷Hz
SLEW RATE: 20V/ms
BANDWIDTH: 8MHz

In fact the 2134 seems to have less spectacular test results. Since you have those TLO-72 op-amps in circuit directly afterwards it is doubtful any sonic change could be heard over those devices. I have seen the TLO-72's replaced by Burr-Brown devices but it also required that the power supply be beefed up to accommodate the higher current draw they made. As I recall it was one of those hybrid mods from those LP after-market folks back east. There name evades my memory tonight.

IMHO save your money or spend it on something more worthwhile then this device. I have worked on them before myself, and the BB modded one from the after-market company, so my opinion while unsettling to some is based on real world handling of several of these...hope this helps some...
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Old 24th January 2012, 08:21 PM   #13
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Okay after doing the traces that were cut, I notice that the left input gain pot had to be almost all the way clockwise while the right input gain had to be all the way counterclockwise just to match the outputs.

Which is not good because now my output signal is low. Then after leaving it on with a sinewave of 2khz the output signal on the "left" op amp was around 6-7v while the right was STEADY at 4v.

So I lower the left input pot almost all the way down to match the right output but the left signal keeps going up and down with different fhz while the right stays steady.

Am thinking the culprit is the left input filter cap which is 4.7uf 50v, no sign of damage but I've dealt with caps that show no damage before.
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Old 25th January 2012, 12:51 AM   #14
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Okay found the real culprit. I follow the traces for the signal before & after the caps and they where the same (so good, no need to replace caps). Still left signal is 1/3 of right signal even though both signals are from the same source and measure from the input RCA's the same. So I knew some where the signal path was being reduced.

The only place left was the relay switch which allows the PA2 to have two source inputs or front and fader (with the movements of some taps inside). After checking both input and outputs for the relay I notice right away that the "right" side was perfect but the "left" side was being reduce badly and was moving continuously.

So I went ahead and added some power to the second remote cable (for second source) and I heard the relay switch but it must not be making good contact because there was no power on the IC's nor the lights where on. After 30 or so clicks I connected the remote for source 1 and re-checked the outputs of the relay and now they where both the same and now both trimmer pots are the same and signal is equal now.

Am going to go ahead and replace the relay since Mouser is a few minutes away, I'll pick it up tomorrow with some other parts.
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